The serine protease thrombin occupies a central role in hemostasis and thrombosis (Tapparelli et al., Trends in Pharmacological Sciences 14:366-376 (1993); Lefkovits and Topol, Circulation 90(3):1522-1536 (1994); Harker, Blood Coagulation and Fibrinolysis 5 (Suppl 1):S47-S58 (1994)). Activation of the coagulation cascade through either the intrinsic pathway (contact activation) or the extrinsic pathway (activation by exposure of plasma to a non-endothelial surface, damage to vessel walls or tissue factor release) leads to a series of biochemical events that converge on thrombin. Thrombin cleaves fibrinogen ultimately leading to a hemostatic plug (clot formation), potently activates platelets through a unique proteolytic cleavage of the cell surface thrombin receptor (Coughlin, Seminars in Hematology 31(4):270-277 (1994)), and autoamplifies its own production through a feedback mechanism.
As a multifactorial protein, thrombin induces a number of effects on platelets, endothelial cells, smooth muscle cells, leukocytes, the heart, and neurons (Tapparelli et al., Trends in Pharmacological Sciences 14:366-376 (1993); Church and Hoffman, Trends in Cardiovascular Medicine 4(3): 140-146 (1993)). Platelet activation leads to shape change and aggregation as well as the synthesis, release and secretion of vasoactive substances and lysosomal enzymes. Endothelial cell activation results in the secretion of stimulatory agents leading to increased vascular permeability and adhesiveness for mononuclear cells, one consequence of which is extravasation of leukocytes at the site of thrombin generation. Thrombin induces fibroblast and smooth muscle cell proliferation suggesting that thrombin plays a key role in lesion development following vascular damage. Enhanced automaticity and prolongation of repolarization have been observed in cardiac myocytes showing sensitivity to thrombin. Normal neuronal development has been shown also to be influenced by thrombin. Thus, inhibitors of thrombin function have therapeutic potential in a host of cardiovascular and non-cardiovascular diseases, including: myocardial infarction; unstable angina; stroke; restenosis; deep vein thrombosis; disseminated intravascular coagulation caused by trauma, sepsis or tumor metastasis; hemodialysis; cardiopulmonary bypass surgery; adult respiratory distress syndrome; endotoxic shock; rheumatoid arthritis; ulcerative colitis; induration; metastasis; hypercoaguability during chemotherapy; Alzheimer's disease; and Down's syndrome.
To date only three classes of compounds (heparins, low-molecular weight heparins and coumarins, such as warfarin) have been used in anticoagulant therapy. Each class has severe limitations and liabilities (Weitz and Hirsh, Journal of Laboratory Clinical Medicine 122:364-373 (1993); Raj et al., The American Journal of the Medical Sciences 307(2):128 (1994)). All three classes indirectly inhibit thrombin. Heparin and low-molecular weight heparins augment antithrombin III and/or heparin cofactor II inhibition of thrombin, whereas coumarins inhibit vitamin K-dependent post-translational modification. Close monitoring and titration of therapeutic doses is required when employing these agents due to patient variability. Hemorrhagic complications due to bleeding are an encountered side effect. In fact, bleeding remains as the most common side effect of long term oral anticoagulant therapy. Lack of activity in arterial thrombosis in the case of heparin is due to its inability to inhibit clot bound thrombin. Lack of oral activity in the case of heparins and low-molecular weight heparins preclude their use for chronic administration.
Direct thrombin inhibitors of various structural classes have been identified recently (Tapparelli et al., Trends in Pharmacological Sciences 14:366-376 (1993); Claeson, Blood Coagulation and Fibrinolysis 5:411-436 (1994); Lefkovits and Topol, Circulation 90(3):1522-1536 (1994)). Representative compounds that act by inhibiting the active site of thrombin include the .alpha.-chloroketone D-phenylalanyl-L-prolyl-L-arginyl chloromethylketone (PPACK), the boroarginine DUP714, the peptide arginal GYK114766, the cyclic peptides cyclotheonamides A and B, the benzamidine NAPAP, and the arylsulphonylarginine argatroban. The thrombin inhibitory peptides hirudin and hirulogs additionally span through the active and exosite domains of thrombin. The peptide hirugen and single-stranded DNA aptamers inhibit thrombin through exosite occupancy.
Experimental studies with direct thrombin inhibitors have shown efficacious antithrombotic effects in various animal models (Lefkovits and Topol, Circulation 90(3):1522-1536 (1994)). Direct thrombin inhibitors may take on an important adjunctive role in thrombolysis and may offer a beneficial role in the field of coronary intervention. Clinical studies with direct thrombin inhibitors for treating acute myocardial infarction, for treating unstable angina, and for patients undergoing diagnostic coronary angiography have provided encouraging results. Nevertheless, these classes of antithrombotic agents still suffer from one or more of the following liabilities: (1) poor oral bioavailability due to the peptidic or oligonucleotidic nature of these agents, or high molecular weight or charged nature of the agents; (2) potential for bleeding complications; (3) poor selectivity towards thrombin versus other serine proteases (which may lead to severe and sometimes fatal hypotension and respiratory depression in animal models); (4) liver toxicity; or (5) cost effectiveness.
A need continues to exist for non-peptidic compounds that are potent and selective inhibitors of thrombin, and which possess greater bioavailability and fewer side-effects than currently available direct inhibitors of thrombin.
U.S. Pat. No. 5,248,673, issued Sep. 28, 1993, discloses bisamidine derivatives as thrombin inhibitors. The patent discloses that these compounds can be used in the treatment of thrombosis, ischemia and stroke.
PCT Published Application WO 93/15756, published Aug. 19, 1993, discloses peptide aldehyde analogs that exhibit thrombin inhibiting activity.
PCT Published Application WO 94/20526, published Sep. 15, 1994, discloses peptide derivatives having a C-terminal boronic acid group. The published application discloses that these protease inhibitory activity.
Nelson et al., NIDA Res. Monogr. 69:204-230 (1986) discloses analogs of morphiceptin, an opioid peptide and the testing thereof as .mu.-receptor agonists and antagonists. The peptide analog L-tyrosyl-N-[2-(4-nitrophenyl)ethyl]-L-prolinamide is disclosed.
Bajusz, Symp. Biol. Hung. 25:277-298 (1984) reviews the structural and inhibitory properties of peptide inhibitors of trypsin-like enzymes, such as thrombin, plasmin, kallikrein and trypsin. The compound D-phenylalanyl-N-[2-[4-[(aminoiminomethyl)amino]phenyl]ethyl-L-prolinamide is disclosed.